1. Technical Field
The invention relates to a feedback light tuning technique and, in particular, to a technique that applies to superfluorescent fiber source (SFS) and reduces mean wavelength drifts caused by environment changes using feedback light tuning.
2. Related Art
In both engineering and scientific domains, optical communication systems are important tools because of their high sensitivity and stability for sensing applications. In particular, they are immune to electromagnetic interference (EMI).
The light source is an essential element in an optical communication system with demands of high output power, broad bandwidth, and stable mean wavelength. Since SFSs doped with rare earth (RE) ions have been developed, the light source can meet the above requirements. In particular, the optical fiber doped with erbium is commonly used.
The SFS is also considered as the most appropriate light source for high precision interference fiber optic gyroscope (IFOG). The IFOG is a tool for sensing the rotational rate based on optical interference. The precision of state-of-art IFOGs has met the requirements of aerospace navigation and become one of the most precise orientation systems. Recently the IFOGs have demands of been employing in the space environment, such as the navigation of the satellites. The primary radiation effect of the SFS is power loss and mean wavelength drift, which is inversely proportional to the scale factor of an IFOG. To meet the requirement of aerospace navigation, the scale factor drift needs below 10 parts per million (ppm). Therefore, it is important to find out how to reduce the mean wavelength drift of an SFS.
The mean wavelength drift of a usual SFS can reach up to thousands of ppm. Consequently, some vendors propose to directly add a filter to the output terminal of the SFS, such as those in U.S. Pat. Nos. 7,142,355, 5,684,590, 6,744,966, and 6,025,915. It is used to restrict the peak output around a limited range (e.g., around 1530 nm). This shrinks the drift (e.g., to 13 ppm). However, this method also reduces the power and bandwidth (e.g., ≦10 nm), unable to satisfy the requirements on the light source for high-precision gyroscopes. Moreover, using the filter to achieve the stability of the mean wavelength of the SFS in the prior art has to sacrifice the output power or bandwidth to some extent. In a radiation environment, the SFS cannot meet the requirements of the light source for high-precision gyroscopes; i.e., high output power, wide bandwidth (30 nm) and stability in the mean wavelength at the same time.
Lately, optical communication systems have demand to be applied in harsh environments, such as those with a large temperature variation or a lot of radiation. Therefore, to develop a technique to stabilize the mean wavelength of the SFS in harsh environments and to increase the range of acceptable radiation dose is an important topic in the field.